There's the case of Bethlehem Ayele, who was driving north on Commonwealth Avenue in Alexandria.

When the 34-year-old woman stopped at the red light at Mount Vernon Avenue, someone came up to her vehicle and shot and killed her. Police think she might have been targeted that October night two years ago.

But they don't have a motive or suspect.

The same is true in the case of Curtis F. King, who was found dead in a parking lot of the Edlandria apartment complex, his body riddled with bullets. Fourteen years later, the mystery lingers.

Then there's the unsolved slaying of prominent real estate agent Nancy Dunning, wife of then-Sheriff James Dunning. She was found shot to death in their Del Ray home in 2003. Police are still searching for her killer.

There's little more frustrating for law enforcement officers than a case gone cold. And because there is no statute of limitations in homicide cases, police keep at it, hoping that one day, with enough persistence or luck or some combination of the two, they'll discover a key piece of evidence, or a once-reluctant witness will come forward.

Having an unsolved case is "very, very frustrating," Alexandria Police Chief David P. Baker said. Murder "affects the whole community and people and families. And from a personal experience, these detectives spend a large portion of their personal and professional time and energy to try to figure these out. They don't forget about them."

That's why Alexandria police have unveiled a Cold Cases page on the department's Web site. The page features four of the city's most trying homicide cases.

"The point I'm trying to make in posting the cold cases is that it's our obligation to look at these," Baker said. "You can solve the case even though an amount of time has passed. The department needs to remain optimistic. And if you hold out hope that something will come forward, you have a greater opportunity to solve the case."

Technologies improve, and police are constantly resubmitting DNA evidence to forensic laboratories for testing on cold cases. That's how Arlington County police were able to arrest a suspect from a 15-year-old abduction and sexual assault case involving a 9-year-old victim. In January, authorities resubmitted DNA and got a hit. In March, they arrested Benjamin Ramirez-Segovia, 43, of Petersburg, Va. On Dec. 5, he pleaded guilty in Arlington County Circuit Court to one count of abduction with intent to defile, one count of rape and two counts of sodomy. He is being held without bond in the Arlington County jail, police said.

In addition to publicizing cold cases, Alexandria police have launched a Most Wanted site, featuring suspects in crimes as different as murder and using stolen credit card numbers. Baker said that in many cases, "it's just a little piece of information that helps solve the puzzle."

Police are investigating the case of Alisha "Lisa" Johnson, who was last seen alive July 28, 1996, in the 600 block of Four Mile Road. Her body was found a week later in Fauquier County.

The fifth anniversary of Dunning's death was this month, but police seem keen as ever to solve the case.

The Cold Cases Web page says: "Detectives are seeking the help of any business, personal friend, associate, acquaintance, or neighbor of Nancy Dunning, in an attempt to reconstruct as much detail as possible regarding her activities and relationships over the past several years. A substantial reward is being offered for any information leading to the arrest and indictment of the person or persons responsible for this Dec. 5, 2003, homicide. This is not a cold case, but an active homicide investigation. Even the slightest detail could be significant."